The present invention relates to the controlled solidification of a material from a liquid in order to obtain a single crystal. It relates in particular to mono-directional solidification in a crucible by Bridgman or xe2x80x9cGradient Freezexe2x80x9d pulling methods. It is particularly well adapted to the growth of compound materials of type II-VI or IV-VI.
According to Bridgman or xe2x80x9cGradient Freezexe2x80x9d methods of directional crystal growth, the product to be crystallised is introduced into a crucible at the bottom of which a seed may previously have been introduced. After the product to be solidified (but not the seed) has been melted in an oven, the next step is to grow the crystal either by mechanically displacing the crucible in a temperature gradient, or by slowly cooling the oven. If the pulling or cooling speeds and the temperature gradient are optimised, a single crystal is generally obtained.
There are however categories of materials capable of applications in single crystal form and for which it is impossible to obtain single crystals since grains and twins appear during crystal growth. These are materials known under the generic names II-VI and IV-VI since they are constituted by alloys of elements belonging respectively to columns II and VI or IV and VI of the Mendeleiev table. The explanation for this phenomenon lies in the fact that, after melting, the liquid includes atom xe2x80x9cclustersxe2x80x9d which, during solidification, are deposited anarchically on the growth interface and modify the crystal orientation. Reference may be made on this subject to the article xe2x80x9cThe crystal perfection depends on the superheating of the mother phase tooxe2x80x94experimental facts and speculations on the xe2x80x98melt structurexe2x80x99 of semiconductor compoundsxe2x80x9d by P. RUDOLPH et al., in Journal of Crystal Growth, 166 (1996), pages 578 to 582.
As the material melts, corresponding to the destruction of the crystalline network, the material is not separated into isolated molecules. These molecules remain cohered in clusters thus giving a certain crystal orientation, which differs from one cluster to another. These clusters accumulate at the bottom of the liquid part, near the solid-liquid interface where they have just been created. If the liquid is re-solidified, the clusters are incorporated into the solid according to a random crystallographic orientation, which constitutes so many sources of disorientation preventing a single crystal from being obtained.
These clusters may be destroyed by superheating the liquid up to a certain temperature above the melt temperature (for example a superheating of 25xc2x0 C. in the case of CdTe). In practice, it is however necessary for all the liquid to be superheated, which is not guaranteed by the natural convection prevailing in the liquid, this convection not being sufficiently strong to displace the clusters located in the colder zone of the liquid, i.e. immediately above the solid-liquid interface.
Using a rotating magnetic field during solidification of the material has been proposed in order to stabilise natural convection in such a way that fluctuations in temperature and associated fluctuations in chemical composition are reduced. Reference may be made in this matter to the articles by P. DOLD and K. W. BENZ entitled xe2x80x9cModification of Fluid Flow and Heat Transport in Vertical Bridgman Configurations by Rotating Magnetic Fieldsxe2x80x9d, in Cryst. Res. Technol. 32 (1997), 1, pages 51 to 60 and xe2x80x9cRotating Magnetic Fields: Fluid Flow and Crystal Growth Applicationsxe2x80x9d in Progress in Crystal Growth and Characterization of Materials (1997), pages 7 to 38.
The present invention makes it possible to overcome current problems in obtaining single crystals of compound materials of type II-VI and IV-VI of electronic quality. It consists in using a rotating magnetic field to displace the atom clusters in a part of the melting bath where the temperature is highest. The clusters melt in this part of the bath while the temperature field in the vicinity of the liquid-solid interface continues to be adapted to growth.
Contrary to the prior art mentioned above, the magnetic field is not used to homogenise temperature and chemical composition in the melting bath. The object of the method according to the invention is in a way to force natural convection instead of stabilising it. To do this, a rotating magnetic field according to the invention is applied before solidification starts while, in the prior art, the magnetic field exerts its action when the bath is depleted of one of the constituents of the mix, i.e. during growth.
The object of the invention therefore is a method for manufacturing a solid single crystal from a material which is electrically conductive in the molten state, by pulling from a molten mass of this material, the material presenting clusters at melt, the method including:
a melt stage so as to obtain said molten mass, the melt stage procuring a colder zone of the molten mass, from which the single crystal will be pulled, and a hotter zone having sufficient temperature to melt the atom clusters,
a stage of application to the molten mass of a rotating magnetic field allowing the atom clusters to be displaced from the colder zone to the hotter zone,
a stage of growth by pulling of the single crystal after the atom clusters have been displaced from the colder zone to the hotter zone, the single crystal being formed from the end of the molten mass located near the colder zone.
To advantage, the melt stage procures a hotter zone representing more than half the volume of the molten mass.
Preferably, pulling is performed along a vertical axis. In this event, the colder zone of the molten mass may constitute its lower part whereas the hotter zone may constitute its upper part.
Preferably, the melt stage is conducted so that there is a steady temperature variation between the temperature of the colder zone and the temperature of the hotter zone.
To advantage, the characteristics of the magnetic field are such that the hydrodynamic boundary layer, located near the liquid-solid interface is reduced as much as possible.
The pulling of the single crystal may be carried out by the Bridgman method or the method known by the name of the xe2x80x9cTravelling Heater Methodxe2x80x9d or THM.
The invention applies particularly to the manufacture of a single crystal of a material selected from materials of type II-VI and IV-VI.